The present invention generally pertains to an ophthalmic lens. More particularly, the present invention relates to a contact lens, and even more particularly relates to a rigid gas-permeable contact lens.
When contact lenses were first commercialized, they were made of a rigid material, such as polymethylmethacrylate (PMMA). As shown in FIG. 1, such contact lenses had a posterior surface 10 including a central optical zone 12 and one or more peripheral zones 14, 16. Optical zone 12 on posterior surface 10 was defined by a generally spherical surface having a radius of curvature corresponding to a measured base curve of the cornea upon which the contact lens was to be worn. The peripheral zones (14, 16) were also defined by spherical curves and were typically flatter than the curve defining central optical zone 12. The peripheral zones were dimensioned to closely fit the outer area of the cornea. The anterior surface 20 of these early contact lenses typically had one to three zones, including a front central optical correction zone 22, an optional intermediate zone 24 and an optional peripheral lenticular zone 26. Both of these anterior surface zones were defined by spherical curves, with the radius of curvature of anterior central optical zone 22 being selected so as to provide the appropriate optical power for correction of the patient's hyperopic, myopic, and/or astigmatic condition. The radius of peripheral lenticular zone 26 on anterior surface 20 was selected so as to provide a transition from central optical zone 22 to an edge 17 of the contact lens, which typically had a thickness of, for example, 0.125 mm. The transitions 25 between zones on anterior surface 20 generally aligned with transitions (18, 28) between zones on posterior surface 10.
In the late 1960's, contact lenses were modified somewhat by removing the discrete junctions (18) between the zones by joining the spherical curves with tangential transitions (28). Subsequently, the posterior surfaces of the contact lens were made aspheric so as to more closely fit the anterior surface of the cornea, which is also aspheric.
While soft contact lenses were developed subsequent to rigid lenses and are currently in wide use, such soft contact lenses cannot be used as effectively to correct the vision of individuals having more severe optical impairment. Thus, there remains a large market for rigid and rigid gas-permeable (RGP) lenses.
Individuals who wear rigid lenses may experience discomfort, which is typically caused by the sensation of having a rigid foreign object with a significant amount of mass on the individual's eye. While consideration has been given to reducing the thickness of the rigid contact lenses so as to reduce their mass and increase oxygen permeability, thin rigid contact lenses have not been commercially feasible due to the weakening of the contact lens structure and the resultant increased likelihood of breakage, warpage, and flexure. Flexure in a rigid contact lens is undesirable because the lens flexes each time the patient blinks thereby resulting in variable vision correction.
Thin lens designs gained worldwide popularity with the introduction of Syntex's Polycon I material in 1979. That material, now coined a silicon/acrylate, had an oxygen permeability (DK) of 5.times.10.sup.-11. The lenses were available in inventory designs of 9.5/8.4, 9.0/7.8, and 8.5/7.3 (outer diameter/optical zone). The 9.5-mm diameter lens incorporated a specific anterior lenticular design and a standard spherical tri-curve posterior design. The lens was fitted approximately 1.00 to 1.50 D flatter than "K" to permit lid/lens attachment in unobstructed upward lens movement. "K" is considered to be the flattest central curve of the cornea.
The Polycon I lens was eventually phased out and replaced with the Polycon II material, which has a DK=12.times.10.sup.-11. Past attempts to produce thin RGP lens designs in moderate to high DK materials have met with only limited success. Material brittleness, warpage, and base curve instability as well as visual interference caused by flexure have prevented the designs from enjoying the success of the low DK Polycon II.
Therefore, a need has existed in the rigid contact lens market for a contact lens having a lower mass while retaining structural rigidity and strength.